Mass Transport and Dynamics in the Earth System...• Earthquakes, volcanoes, landslides, tsunamis...
Transcript of Mass Transport and Dynamics in the Earth System...• Earthquakes, volcanoes, landslides, tsunamis...
Mass Transport and Dynamics in the Earth System
Richard S. Gross���
Jet Propulsion Laboratory California Institute of Technology Pasadena, CA 91109–8099, USA
19th International Workshop on Laser Ranging
October 27–31, 2014 Annapolis, Maryland
Forcing Mechanisms • External • Tidal potential • Body, ocean, atmosphere
• Surficial • Atmospheric winds and surface pressure • Oceanic currents and bottom pressure • Water stored on land (liquid, snow, ice)
• Internal • Earthquakes and tectonic motions • Mantle convection • Outer core dynamics / coupling with mantle • Inner core dynamics / coupling with outer core and mantle
Geodetic Measures of Earth’s Response • Rotation • Angular momentum exchange (surface fluids, outer core) • Torques (external, surficial, internal) • Changes in inertia tensor (earthquakes, GIA)
• Gravity and geocenter • Mass distribution (static field) • Mass redistribution (time varying field) • Surface displacements (ground-based)
• Shape • Tidal displacements • Surface loading and unloading • Internal deformation • Earthquakes, core pressure => mantle deformation
Selected Scientific Questions • Climate change • Global warming • Atmosphere, oceans, Earth surface
• Ice mass balance / sea level change • Mountain glaciers, grounded and floating ice sheets
• Natural hazards • Earthquakes, volcanoes, landslides, tsunamis
• Earth structure • Interior figure, mantle anelasticity, 3D mass distribution
• Mantle and core dynamics • Coupling mechanisms
• Origin of observed variations • Earth rotation, gravity and geocenter, shape
Pathways to Advancement • Improved measurements
• Accuracy, temporal resolution, duration
• Improved models • Forcing mechanisms • Atmosphere, oceans, hydrology, core
• Earth’s response to forcing • Anelastic effects
• Improved theory • Higher order • Dynamic oceans and core • Core-mantle coupling
• Consistent computations • Love numbers
Earth Rotation Theory • Assumptions made in current theory • Equations of motion have been linearized • Terms smaller than 1 part in 300 discarded • Separates polar motion from length-of-day variations (no spin-wobble coupling) • Earth is dynamically axisymmetric (A = B << C) • Chandler wobble is circular, not elliptical (semimajor/minor axes differ by few cm)
• Oceans respond passively to wobble • Not valid for wobbles of period < ~10 days
• Core responds passively to wobble • Core assumed to be homogeneous, incompressible, non-dissipative • Core is uncoupled from mantle
• Theory is accurate to about 1 part in 300 • Measurements are more accurate than this
• IAU/IAG JWG on Theory of Earth Rotation formed
Consistency of Computations • Example: Love numbers • Computations from different groups differ by a few percent • For same Earth model
• IERS Conventions • Helped impose consistency in data reduction procedures
• GGOS Conventions (?) • Would help impose consistency in geodetic computations
Period and Q of Chandler Wobble
Gross (2014)!
Constraints on Mantle Anelasticity
Benjamin et al. (2006)!
Trend in Pole Path
Chen et al. (2013)!
Decadal Polar Motion Variations • Observed excitation
• COMB2010 • Combination of optical astrometric, LLR, SLR, VLBI, and GPS observations • Spans 1962.0 – 2011.5 at daily intervals
• Modeled excitation • Helmholtz Centre Potsdam • Consistent estimates of AAM, OAM, and HAM • AAM computed from ECMWF • OAM computed from Ocean Model for Circulation and Tides (OMCT) • HAM computed from Land-Surface Discharge Model (LSDM) • Ocean and hydrology models driven by ECMWF • Global atmosphere/oceans/hydrology mass conservation imposed
• Detrended & low pass filtered • Cutoff period of 6 years
Future Research Directions • Extend theory (IAU/IAG JWG on ThER) • Triaxial, deformable body with fluid core & dynamic (!) oceans
• Improve existing models • Oceanic (tidal and non-tidal) • Mass, not volume, conserving non-tidal OGCMs • Hydrologic • Daily values • Core-mantle interactions
• Develop new models • Present-day ice mass / sea level change
• Signal of unknown origin • Markowitz wobble • Quasi-periodic wobble on decadal time scales; amplitude of about 30 mas • Caused by terrestrial water variations? Ice sheet mass / sea level? Core-mantle?